IDL compiler: sync Python to r167755

Python side to:
Have DOMImplementation::document() return reference.
https://codereview.chromium.org/140653013/

TBR=haraken
BUG=239771
NOTRY=true

Review URL: https://codereview.chromium.org/180253007

git-svn-id: svn://svn.chromium.org/blink/trunk@167844 bbb929c8-8fbe-4397-9dbb-9b2b20218538